I think they're a bit different from Dansk Folkeparti (DF), mostly in having considerably more questionable roots, which I would guess doesn't help their acceptance in the party mainstream. They have made an effort to clean up their image in the past few years, expelling a number of the extreme-right elements, but the party's roots in the far-right scene makes them a bit anathema to respectable society. The origins of the party, and much of their '80s and early-'90s crowd, came out of the "subcultural right", movements like
http://en.wikipedia.org/wiki/Bevara_Sverige_Svenskt, norse-pride and white-power metal bands, the motorcycle club scene, etc.
When Mikael Jansson defected from the Center Party and took over Sweden Democrats in 1995 he began a purge of the openly extreme-right elements of the party in order to turn it into a respectable anti-immigration movement, but it didn't really solidify until the past few years. Even in recent years they have had a struggle to keep those types out of the party, for example their youth leader from 2007-2010 (Erik Almqvist) was embarrassingly videotaped singing white-power music at a retreat, and was finally forced out of the party earlier this year (weirdly, for someone who opposes immigration, he reportedly emigrated to Hungary after resigning).
DF by contrast has different origins, culturally rooted in something more like old-person Danish culture, rather than youth radical-right culture. They have a vaguely mythologized idea of 19th-century or early-20th-century Danish culture, as they imagine one's grandfather in a small town in Jutland might have lived. They generally therefore view the radical right-wing scene as negative, as equally a deviation from traditional quiet Danish values as immigrant crime is. This has let them keep those elements out of the party more credibly, because it's not only for image that they do it, but because their base actually culturally dislikes the far-right (especially anyone associated with the metal or biker scenes, who they see as culturally un-Danish).
Not that I myself would vote for DF (as an immigrant in Denmark they aren't out to make life easier for me), but I somewhat understand their appeal better; they are more of a rural "traditionalist values" party than a radical nationalist party. I find it harder to understand how SD would get similar support, unless people either don't realize their association with the far-right subculture, really believe that their transformation is completed, or are just lodging protest votes.